1. Field of the Invention
The present invention relates to radar guidance systems. More specifically, the present invention relates to systems and methods for measuring line-of-sight angular rates for guided projectiles.
2. Description of the Related Art
Navy ships are exposed to low flying, fast, and highly maneuverable missile threats. In order to provide the ships with an effective missile defense system, high accuracy measurements of incoming missile targets and precision guidance of anti-missile projectiles are required.
Many guidance systems have been developed for missiles and projectiles (i.e. xe2x80x94bullets). In a typical radar based guidance system, the projectile is guided to the target by guidance signals developed from tracking data obtained either by a shipboard radar system or by a radar system located totally, or partially, within the projectile. The former system is commonly referred to as a command guidance system and the latter as a homing guidance system.
In a command guidance system, a high-resolution shipboard radar system racks both the target and the projectile, calculates the proper guidance signals for the projectile based on the generated tracking data, and transmits the signals to the projectile to enable the projectile to intercept the target.
In a homing guidance system, the target tracking radar system is located totally or partially within the projectile. An active homing guidance system uses a monostatic radar system where both the radar transmitter and receiver are located in the projectile. A semi-active guidance system uses a bistatic radar system where a radar transmitter located remotely from the projectile (such as onboard the ship) illuminates the target and the reflected returns are received by a receiver located on the projectile. The tracking data from the radar measurements are then used to calculate the proper guidance signals to direct the projectile to the target.
Most of these systems are designed for use with missiles and larger caliber projectiles (greater than 3 inches in diameter), whereas the optimum caliber for high rate-of-fire guns is generally about 1 inch in diameter. Prior art guidance systems do not work well with smaller caliber projectiles. In particular, prior art approaches do not accurately measure the line-of-sight angular rate to the target with enough precision for the application. Command guidance systems with a high resolution monostatic shipboard radar are capable of measuring line-of-sight angular rate. However, these measurements are generally not as accurate as measurements made from the projectile, as with homing guidance systems. Homing systems, however, require a radar receiver onboard the projectile. The size of the smaller caliber projectiles places a constraint on the size of the radar receiver antenna on the projectile. With a small antenna, a relatively accurate range rate can be measured, but the angular rate will be imprecise.
The critical factor required for effective projectile guidance is an accurate measurement of the line-of-sight angular rate to the target relative to the projectile. Guidance algorithms depend on line-of-sight angular rate information to successfully direct a projectile to its target. Poor line-of-sight angular rate measurements may cause a projectile targeting error.
Hence, there is a need in the art for an improved method or system for accurately measuring line-of-sight angular rates for precision guidance of small caliber projectiles.
Furthermore, these guidance systems need to be effective under all weather conditions. Laser or ladar based guidance systems have been developed for small caliber projectiles. These systems offer very high angular resolution; however, they typically require favorable weather conditions to be effective. Adverse weather such as fog, rain, or clouds may block optical electromagnetic energy, causing a laser based guidance system to fail.
Hence, a need exists in the art for an improved method or system for accurately measuring line-of-sight angular rates for all-weather precision guidance of small caliber projectiles and a guidance system based thereon.
The need in the art is addressed by the system and method for measuring line-of-sight angular rates for all-weather precision guidance of projectiles and the guidance system based thereon of the present invention. This invention takes advantage of the fact that several projectiles are usually fired at the incoming target. While the LOS angular rate cannot be determined solely from the range rate measurements from a single projectile, it can be calculated if the range rate information from several projectiles is available.
In accordance with the novel method for measuring line-of-sight angular rates, first the range rates of the target relative to at least two projectiles is determined, as well as the position and velocity of each projectile. Then, the line-of-sight angular rate of the target relative to at least one projectile is computed from the range rates, positions, and velocities. In the illustrative embodiment, the range rate of the target relative to a projectile is determined based on a monostatic target Doppler measurement, a monostatic projectile Doppler measurement, a bistatic Doppler measurement of the target by the projectile, and the carrier frequency of a data link between the projectile and the shipboard system.
In an illustrative embodiment, the guidance system of the present invention includes a monostatic radar illuminating the target, bistatic receivers aboard at least two projectiles fired at the target, and a system for determining line-of-sight angular rates to the target based on the monostatic measurements and the bistatic measurements from at least two projectiles. The guidance system further includes a system for computing guidance command signals for at least one projectile based on the line-of-sight angular rates, and a projectile steering unit aboard at least one projectile for steering the projectile based on the guidance command signals.